Devices of this type are, in addition to other uses, used for pressure measurement in aggressive media. The dividing membrane is contacted with an aggressive first medium and the pressure prevailing in the first medium is transferred to a second medium in the pressure chamber. There, the pressure is conducted either immediately, or over a suitable pressure conducting line, to a pressure measurement cell.
An incompressible hydraulic liquid, especially a hydraulic oil, is preferably used as the second medium for pressure transmission. In so far as such a substance is suited for contaminating a medium being measured, it is necessary to recognize early that there is a leak in the dividing membrane, or a rupture of the dividing membrane. To this end, Bastiaan discloses in the European patent application EP 0 838 672 a pressure transmitter, in which the pressure chamber has in its wall a dead-end hole, in which the electrode of a conductivity sensor is arranged. In such case, when the dividing membrane breaks, the first medium diffuses into the pressure chamber and into the dead-end hole, whereupon a change in the conductivity is detectable, provided that the first medium has a conductivity which deviates significantly from that of the second medium. This is especially the case, when the first medium is an aqueous medium and the second medium is a hydraulic oil, especially a silicone-based oil.
The described device has, however, the disadvantage that it ascertains a failure of the device only after the membrane rupture has already occurred and without having given any anticipatory warning that an exchange of the sensor was necessary. This behavior frequently leads to unacceptable down times for the installation. In order to prevent this, these pressure transmitters have to be exchanged on a precautionary basis during routine maintenance after a theoretical maximum lifetime, with corresponding factor of safety, has been reached. This can, however, lead in many instances to a premature exchange of the device, when actual membrane corrosion has not yet been so extensive that an exchange after the mentioned time is really needed.